Technical approaches

GUI

The nanoGUI we used in this project is adapted from last course project. Besides the commands P/N/R implemented already, we add command S for turning on/off the stop mode.

• P: pause/continue
• N: next time frame
• R: reset
• S: turn on/off stop mode

Point mass model

We represent every bird as a point mass, and there are three forces coherence, separation, and alignment between them. The implementation is based on boids algorithm. However, with this implementation, the speed of birds will go crazy when we increase the weights of the forces and then decrease it back to the original value. Therefore, we added a maximum speed for the birds to make sure the speed is always reasonable.

• Coherence force cause acceleration in the direction of average position of nerghbor birds.
• Separation force cause acceleration in the opposite direction of average position difference between the bird and its neighbor birds.
• Alignment cause the velocity of the bird increase by the average velocity difference between its neighbor birds' and itself.

Then we add some interative features, so user can use sliding bars on GUI to change:

• Weights for the three forces (the larger the weight, the more obvious the behavior is)
• Neighborhood size for the three forces (radius for the circle)
• flock size (number of point masses)
• heading position (general direction birds will fly to)

Bird model

To further picture the details of birds, we build a 3D build model using Blender. By aligning the model axis with the velocity direction, we can make sure birds fly with their head facing the front. We also manually painted a texture to add more details like feathers beak, and claws.

Stop mode & Tree model

To include more realistic and interesting behaviors, we also added stop mode that the birds can stop at a random position on the nearest branch if it's close enough to the branch.

We start by adding ground and a single pole to the scene. Every time when the distance between a bird and the pole is within a fixed value, the bird will fly towards a random position on the pole calculated by implicit equation with deceleration until it rests on the pole. According to probability theory, the birds should have even spacing between them. After some fixed time, birds will take off again randomly with a probability according to Russian Roulette. After we see everything works well on the single pole, we added more branches and make a tree via rotation and scale. The birds now will stop on the nearest branch.

Lessons learned

To conclude, what we learned from lectures and put in this project includes:

• How to use particle system to represent flocks as point masses.
• How to simulate the movements and interaction between birds using forces according to Newton’s Law and Euler’s Method.
• How to use polygon mesh to make models.

Besides, it's always important to set restrictions according to physical conditions. For example, we know the bird speed must has an upper bound, and the speed change is continous and smooth, so there must be deceleration when birds try to stop on the branch.